Borg Warner transfer box, driving without front prop

[Snagger]

Hi all.

I just wanted to double check my understanding of how the viscous unit and output shafts of the BW transfer boxes work.  As I understand it, the chain turns the outer casing of the diff unit and that drives the rear output solidly, but the front output is via the viscous unit.

If so, that enables driving with the front prop removed (carefully so as not to overload the rear axle), but driving on the front axle only would result in a lot of slip of the viscous unit and damage in pretty short order.

Is that right, or does the viscous unit behave more like an axle diff (in this case an LSD) in having the chain turn the outer diff shell, but both output shafts being driven by the diff internal gears and both subject to slip compared to the shell?

[Bowie69]

Rear drive -as you say. Fine for quite a while to do this.

Front drive -the coupling will slip but then lock up and should give you drive. Wouldn't really recommend for more than just a quick test.

[Escape]

The diff in the Borg Warner is a proper one, with the drive to the carrier, and both outputs connected via bevel gears and the viscous coupling. So you can drive in either RDW or FWD by removing the other propshaft, but in both cases the viscous will have to take the full load. Fine to get you home, but don't drive like that any longer then necessary. Unless the viscous is already seized.

[Snagger]

Unfortunately, I read those as conflicting answers!

I was under the impression from the sectional drawings that BW has a conventional diff with a viscous unit inside making act as a limited slip diff but without the plate wear.  However, seeing photos of the viscous unit made me think it was comparable in operation to that on the Freelander, just contained within the transfer box rather than external.  Maybe the sectional diagram I saw was for an LT230 and mislabelled.  This is what I’m trying to get to the bottom of…

I managed to find an online workshop manual with diagrams and description.  Basically it is an LSD with both outputs from the “side gears”, as per an axle.  It was the configuration with the viscous unit mounted in front of the diff rather than inside it that threw me, but a close look showed the concentric shafts that allow function just like a classic LSD.  I presume they configured it that way to allow a much bigger viscous unit than conventional clutch pack.

So, it means that driving on a single axle will likely slip the viscous unit unless driven extremely gently and not have the solid drive to the rear I had hoped.

[Escape]

You do need to look carefully at the drawing, the concentric shafts are easy to miss. I read it the same way as you do, and that matches the description as well.

[Snagger]

My post above was edited to add the second half before it got any replies, which is why the second half mismatches the first.  I should have added “edit”.

Glad that the resident RR expert sees it the same as I have finally deduced - at least that gives me confidence that I now fully understand the system.  I had hoped that it’d be safe to drive to MoT tests with the front prop removed as they invariably want to use rollers rather than Tapley gauges - I have had that argument with several, but they almost all insist on the rollers.

[elbekko]

Tell them to set their rollers to turn opposite directions, it's fine then. That's what they do here when you mention permanent four-wheel drive.

[Escape]

As above, if they have that possibility. Otherwise, if it's not too far and you drive gently, you should be fine with the front prop removed.

As an aside, removing a prop would be a fail over here, because it is considered a modification of the driveline without consent from the manufacturer and the vehicle no longer complies with it's certificate of conformity...

 

[Snagger]

Good suggestions.  Snag for me is that when I get that RR going again, I intend for it to have ATBs in both axles, but with open diffs the opposing rollers suggestion is golden - it wouldn’t have occurred to me and I had no idea they could do that.

The MoT station I used to frequent was only 2-3 miles from home, so a very gentle drive there may be ok on one axle. Worst case, I could explain the situation and remove the prop on their lift immediately prior to the test - they were always friendly and helpful, so shouldn’t mind five minutes work before and after for a couple of beers.

[David Sparkes]

I think this is relevant, as it involves only using one centre diff output.

In 2004 the rear diff in my diesel powered 38A broke. As designed there was no traction control on either axle, but there was 4 wheel ABS.

To minimise the chance of collateral damage I removed the rear propshaft, then continued my daily commute, about 50 miles a day, so 250 / 300 miles in total.

 This 2WD running lasted about a week while I ordered a replacement differential.

I naturally kept my speed & acceleration demands to a minimum, while keeping up with traffic.
The biggest impact was on traction, the front suspension was in no way optimised for traction, as a front wheel drive car would be.
It was very easy to spin a front wheel exiting junctions, especially if the tarmac was wet.

The handbrake didn't work effectively on slopes, as the VC slipped slightly.

I didn't do any flash things like measuring oil temperatures, just drove it sensibly within the new parameters.
I changed the differential and things returned to normal, the broken diff (cross shaft in three pieces) was sent to Ashcrofts as part of the exchange.

Regards.

[Snagger]

That is relevant.  Did the VC give any trouble after that?

[David Sparkes]

The 'executive summary' has to be No, and Yes.
No, There were no correlated side effects.
By the time I stopped using the vehicle it was showing signs of (what I understood to be) the standard signs of VC decay. Had I still been enthusiastic about the 38A I would have changed the VC, regarding it not as a failure but as a long term service item.

I have seen a photo of a VC split apart, but I suggest only Ashcrofts could make an assessment of the failures seen on exchange items.

More detail.
I bought the vehicle privately in August 1999 at 58,000 miles.
The differential failure occurred in June 2004 at about 143,000 miles.
I stopped using the vehicle in  November 2009 at about 163,000 miles.

My understanding, then and now, is that there are several stages of noticeable signs of VC 'wear'.
One is that on low friction surfaces the ground will be noticeably disturbed when making full lock manoeuvres, even at low speed.
As full lock manoeuvres at low speed on a low grip surface (grass) were normal when leaving or accessing my property this is what I noticed. I have no record of date or mileage.

The second is that the tyres show asymmetric wear, the UK Offside rear being prone to cupping. I didn't notice this, but didn't go looking for it either. There seemed to be no effect on tyre change intervals. As I believe this cupping is due to uneven power (torque) delivery, and the asymmetric suspension, I do not think this uneven wear pattern is affected by whether the vehicle is Left or Right hand drive.

The third effect, if the appropriate service is not carried out, is transmission lock up. Drivers and owners without Series LR experience will notice the vehicle rising as it is driven, and becoming heavy to steer, especially in supermarket car parks (for example). The vehicle will settle down when left overnight, only to rise again the next day.

That example was from a Classic owner. I advised removing the front propshaft, with caution as a front wheel will spin as the tyre adhesion breaks on being jacked up. The owner reported back that the wheel did spin, but didn't shake the axle off the jack head. The improvement in steering was immediate (the wife could go back to doing the shopping). He later advised that while the vehicle no longer rose while driving, it hadn't completely settled down to its previous height.
I was puzzled at the time, but later realised that the tension imposed by the lock up may well have bent the 'ears' by which the Classic radius arms are attached to the chassis. I had no contact with the owner so couldn't pass this thought on to him. Nor did I think he would have attempted rectification. I had no information about whether he changed the VC.

Regards.

[Escape]

Good point about the handbrake being much less effective with the rear prop removed! Definitely a consideration for MOT.

As for breaking traction in FWD, I get that every time I drive a FWD car in the rain... The P38 being a heavy car with (usually) tyres that are not optimized for road driving and a low geared autobox that puts a good amount of torque on the wheels will certainly be no better. But should not be a problem if driven gently (as is needed to save the viscous anyway).